There are possible reasons:
A.Some of the fonts do not map the corresponding code point (aka.missing character)
B.Some fonts map the corresponding code point but there are not valid glyphes.
C.Some of the Chinese can only shows in Simplified style or in Traditional style.If there are simplified Chinese in a Traditional Chinese, it will not show or show as default glyphe.All you need to do it transfer it to the other style.
D.Some fonts have some problems of compatibility causing mistakes but it does not mean that the font is broken.